Geoscience Reference
In-Depth Information
south pole, waves are much less conspicuous and the
mean flow is almost circular.
Many experiments have been conducted to determine
the reasons for this pattern. Clearly surface features play
an important part, even at this height. The presence of the
Rocky Mountains and the Himalayas is believed to 'lock'
the troughs at 80
depression and move more rapidly, perhaps up to 15
longitude per day. The longer waves - usually between
four and six are apparent - move more slowly and can
even retrogress against the westerly flow. They are linked
with the major circulation features such as the subtropical
highs and Icelandic lows. The long-wave flow tends to
'steer' the shorter waves, moving them northwards when
ahead of a trough and southwards when to the rear of a
trough.
E respectively. The distribu-
tion of land and sea is also thought to be of importance.
In the southern hemisphere there are no mountain
ranges of comparable size, nor such marked land and sea
temperature contrasts, so the flow is more zonal.
On a shorter time period, other waves may exist in the
upper westerlies, though their shape is less regular. The
smaller waves tend to be associated with an individual
W and 140
Index cycle
It has been argued that the Rossby waves do alter their
amplitude and wavelength in a roughly cyclical manner
over a period of between three and eight weeks. If we
El Niño-Southern Oscillation
NEW DEVELOPMENTS
In addition to the major north-south exchanges represented by the Hadley cell, we find a major east-west exchange
in the tropical Pacific Ocean which has been called the Walker circulation. The normal situation is a strong flow from
the subtropical high-pressure cells, emphasized by subsidence over the cool ocean currents near South America
(
Figure 6.21
).
Over Australia, the archipelago of Indonesia and the warm Pacific Ocean area, air tends to be rising
and precipitation is abundant. Periodically this circulation is transformed by the cool Humboldt current off Peru being
disrupted and replaced by much warmer waters. As a result, the normally dry areas are wetter as subsidence stops
and the wet areas are drier as subsidence zones shift to dominate these locations. This state of affairs is known as
an El Niño or an ENSO (El Niño-Southern Oscillation) event. Climatologists have derived an index termed the Southern
Oscillation Index (SOI) based on pressure values at Tahiti in the Pacific and Darwin in northern Australia to represent
the state of the circulation (
Figure 6.22
).
Not only does ENSO have a major regional impact in the Pacific, its influence extends to other parts of the world
through the interaction of pressure, air flow and temperature effects. During the major El Niño of 1997/8 there were
climatic extremes in many parts of the world. Australia, southern India and southern Africa had major droughts, but
the greatest effects were noted in Indonesia, where extensive forest fires rampaged, producing vast amounts of
smoke pollution. Levels of particulate matter in the air reached well above the World Health Organization's
recommendations and aircraft flights were affected through poor visibility and air quality. Tropical storms followed
anomalous tracks, reducing rainfall in areas which normally experienced the storms, such as north-east Australia,
and affecting areas outside the usual range, such as Hawaii. In the northern hemisphere California suffered major
storms as the westerly circulation became more intense in the north Pacific.
Increasingly climatologists have begun to realize that anomalies in some parts of the globe can exert an influence
on other parts. These effects have been termed atmospheric teleconnections.
In 1999 the reverse state developed (termed La Niña), with colder than average sea surface temperatures in the
eastern Pacific. This produced further anomalies of temperature and precipitation but in different areas and of different
magnitudes. Much of Australia had above-average precipitation, with water partially filling Lake Eyre, and much of
southern Africa was wet, including the severe floods in Mozambique referred to earlier.
Its causes are not fully understood though it is believed that the onset of an ENSO event develops through complex
oscillations in a dynamic ocean-atmosphere system. Predictions about future El Niños are made on the basis of what
is understood, with some elements of success at least for the short-term forecasts.
